Coin validators which discriminate between coins of different denominations are well known and one example is described in our GB-A-2 169 429. This coin validator includes a coin rundown path along which coins pass edgewise through a sensing station at which coils perform a series of inductive tests on the coins to develop coin parameter signals which are indicative of the material and metallic content of the coin under test. The coin parameter signals are digitised so as to provide digital coin parameter data, which are then compared with stored data by means of a microprocessor to determine the acceptability of otherwise of the coin under test. If the coin is found to be acceptable, the microprocessor operates an accept gate so that the coin is directed to an accept path. Otherwise, the accept gate remains inoperative and the coin is directed to a reject path.
The stored data is representative of acceptable values of the coin parameter data. The stored data in theory could be represented by a single digital value but in practice, the coin parameter data varies from coin to coin, due to differences in the coins themselves and consequently, it is usual to store window data corresponding to windows of acceptable values of the coin parameter data. The width of the windows is a compromise between a number of factors. In order to achieve satisfactory discrimination between true and false coins, the window widths should be made as narrow as possible. However, if the windows are made too narrow, there is a risk that true coins will be rejected as a result of minor differences between the characteristics of true coins.
Another problem is that the window data needs to vary from validator to validator due to minor manufacturing differences that occur between validators manufactured to the same design. Consequently, it is not possible to program a fixed set of window data into mass produced coin validators of the same design. A conventional solution to this problem is to calibrate coin validators individually by passing a series of known true coins of a particular denomination through the validator so as to derive test data from which appropriate window data can be computed and stored in the memory of the validator. Reference is directed to GB-A-1 452 740. This calibration method is however time consuming because a group of test coins for each denomination needs to be passed through the validator in order to derive data from which the w windows can be computed. Alternative techniques are disclosed in WO94/04998 and U.S. Pat. No. 5,067,604.
The present invention provides an alternative approach which allows a single set of window data to be used for all coin validators for a particular design, notwithstanding differences in their characteristics that arise within normal manufacturing tolerances, from validator to validator.